2 files changed, 43 insertions, 4 deletions
diff --git a/chapter10/src/main/java/com/seaofnodes/simple/node/SubNode.java b/chapter10/src/main/java/com/seaofnodes/simple/node/SubNode.java
index dfb4744d..eeec5e1d 100644
--- a/chapter10/src/main/java/com/seaofnodes/simple/node/SubNode.java
+++ b/chapter10/src/main/java/com/seaofnodes/simple/node/SubNode.java
@@ -37,13 +37,24 @@ public class SubNode extends Node {
 
     @Override
     public Node idealize() {
+        Node lhs = in(1);
+        Node rhs = in(2);
+        Type t1 = lhs._type;
+        Type t2 = rhs._type;
+
+        // Keep invalid operands for type checking.
+        if( t1 instanceof TypeInteger x && t2 instanceof TypeInteger y ) {
+            if( y.isConstant() && y.value()==0 ) return lhs;
+            if( x.isConstant() && x.value()==0 ) return new MinusNode(rhs);
+        }
+
         // x - (-y) is x+y
-        if( in(2) instanceof MinusNode minus )
-            return new AddNode(in(1),minus.in(1));
+        if( rhs instanceof MinusNode minus )
+            return new AddNode(lhs,minus.in(1));
 
         // (-x) - y is -(x+y)
-        if( in(1) instanceof MinusNode minus )
-            return new MinusNode(new AddNode(minus.in(1),in(2)).peephole());
+        if( lhs instanceof MinusNode minus )
+            return new MinusNode(new AddNode(minus.in(1),rhs).peephole());
 
         return null;
     }
diff --git a/chapter10/src/test/java/com/seaofnodes/simple/Chapter04Test.java b/chapter10/src/test/java/com/seaofnodes/simple/Chapter04Test.java
index 2e2abb25..f1603cc4 100644
--- a/chapter10/src/test/java/com/seaofnodes/simple/Chapter04Test.java
+++ b/chapter10/src/test/java/com/seaofnodes/simple/Chapter04Test.java
@@ -51,6 +51,34 @@ public class Chapter04Test {
     }
 
     @Test
+    public void testSub0() {
+        Parser parser = new Parser("return arg-0;");
+        StopNode ret = parser.parse();
+        assertEquals("return arg;", ret.print());
+    }
+
+    @Test
+    public void test0Sub() {
+        Parser parser = new Parser("return 0-arg;");
+        StopNode ret = parser.parse();
+        assertEquals("return (-arg);", ret.print());
+    }
+
+    @Test
+    public void testSubNeg() {
+        Parser parser = new Parser("return 3--arg;");
+        StopNode ret = parser.parse();
+        assertEquals("return (arg+3);", ret.print());
+    }
+
+    @Test
+    public void testNegSub() {
+        Parser parser = new Parser("return -arg-3;");
+        StopNode ret = parser.parse();
+        assertEquals("return (-(arg+3));", ret.print());
+    }
+
+    @Test
     public void testVarArg() {
         Parser parser = new Parser("return arg; ");
         StopNode stop = parser.parse();